Making More Bronze (Session 7)

(cracked mold with a drop of bronze inside)

For our first session this year (the weather has not been cooperative), our  plan was to make more bronze because we're almost out and we have lots of plaster molds (made over the winter) to fill. So we loaded the crucible with copper and tin, 90:10.

in the hope of making bronze to cast it in our patiently-waiting molds.

(NB: I realized later, though, that our 90:10 calculation wasn't quite right. We measured the 10% tin in relation to the amount of copper we had, but I think it should be in relation to the final amount of bronze we'll end up with. So we may in fact have been short about 6-7 grams of tin for a true 90-10 ratio. Would that be significant? Would it have helped bring the liquidus temperature lower?)

Many of the molds were cracked, as you can see. It was more than likely that they got too hot too quickly in the kiln, during the wax burnout earlier this year. (The sprues are covered or stuffed to keep them clean before the bronze goes in.)

(this one fell apart completely)

So we only had a few good molds, but we thought we'd try to fill some of the cracked ones anyway, just to see what would happen. We put the crucible in the furnace at 12:15 and covered it with charcoal. 

 

It was difficult to make bronze on a previous occasion, but we managed it with the bellows. We let the furnace heat up on its own for a time and then got the bellows going. 

The trick with this furnace is that we need to find the balance between increasing the heat with the bellows and consuming too much charcoal. Because the furnace gets super hot but the charcoal gets used up quickly, and every time you add more the temperature drops, of course, while you get it all back up to temperature.

I think there's also a sweet spot in terms of crucible placement. Placing it lower (but not too low) helped last session. 

Whatever the precise factors required to reach copper-melting temperature, we didn't quite make it. Or we did, but it wasn't really hot enough to pour because only a little came out. (It didn't help that I hadn't practice handling the tongs, because I couldn't get the crucible tilted sufficiently!)

In the end, all that we poured ended up forming this little ball at the bottom of one of the molds!

But if there had been more molten bronze, the cast might have worked out fairly well - even with the mold being cracked. There'd be a lot of finishing to do, of course.

This is the "bronze" metal chunk that came out of the crucible, later. 

Bits of bronze are visible, along with bits of charcoal and a whole lot of grey which I suppose is badly mixed copper and tin. Next time: we'll try to melt this chunk down and get it cleaned up before casting it into our still patiently-waiting molds.

And we need to learn about calculating the ratio of copper to tin.